This invention relates to spark plug electrodes and more particularly to an improved method for producing a composite center electrode for a spark plug and to a center electrode produced by the method.
The prior art teaches various methods for producing composite center electrodes having an outer shell formed from a corrosion-resistant metal, such as a nickel alloy, and a core formed from a metal having a high thermal conductivity, such as copper. By providing a thermally conductive core to the center electrode, the firing tip of the spark plug operates at a lower temperature. The lower electrode temperature allows the spark plug to be operated at higher specific outputs without causing preignition.
One prior art method for forming a spark plug center electrode having a high thermally conductive core involves first forming a cup from a corrosion-resistant metal. The cup has a closed end and a tubular wall extending upwardly from the closed end to an open end to define a cavity extending centrally therein. A composite billet then is formed by positioning interiorly of the cup a close-fitting right circular cylindrical billet of a metal having a high thermal conductivity. The billet fits tightly within the cup walls to form the composite billet. The method further involves inserting the closed end of the composite billet into the upper end of a bore in an extrusion die. The billet is moved through the bore until it contacts an extrusion orifice of reduced diameter relative to the upper bore and pressure is applied through a plunger to force all except a terminal portion of the billet through the extrusion orifice to form an electrode blank. The electrode blank has the unextruded terminal portion as an upper headed portion, a lower portion of reduced diameter extending longitudinally therefrom and a copper or other thermally conductive metal core extending longitudinally therein. After the electrode blank is removed from the die, it is suitable for use as a composite center electrode in a spark plug. Such a process is illustrated, for example, in the U.S. Pat. No. 3,857,145.
In a modified prior art method for making a composite center electrode, a cup formed from a corrosion-resistant metal is partially filled with a high thermally conducting metal, such as copper, to form a composite billet. The copper is in contact with a closed end of the cup and is spaced inwardly from an open end of the cup. After the composite billet is formed, the edges on the open end of the cup may be turned slightly inwardly to retain the copper within the cup. The composite billet then is inserted into the bore of an extrusion die, closed end first, and pressure is applied through a plunger to force all except a terminal portion of the billet through an extrusion orifice. During the extrusion process, the corrosion-resistant metal at the previously opened end is closed over the thermally conductive core to form a terminal end on the electrode blank. After the electrode blank is removed from the die, the terminal end is further shaped on a header machine and is trimmed, as necessary, to form a terminal having a desired shape. Since the terminal end is formed completely from nickel alloy or some other corrosion-resistant material used for forming the outer surfaces of the electrode blank, a metal terminal wire is easily welded to the terminal end to complete manufacture of the electrode blank.
Both of the above-described methods for forming a composite center electrode for spark plugs initially pass the closed end of the composite billet first through an extrusion orifice. In both cases, the firing end of the center electrode can have a relatively large mass of the corrosion-resistant metal. Above this relatively large mass, the copper or thermally conductive core begins first at a relatively small diameter and increases to a larger diameter in a direction moving along the length of the electrode. Consequently, there is a relatively long heat transfer path from the lower end of the center electrode to the point at which the heat conducting core has a large diameter. Another problem ocurring in these prior art methods is in excessive wear on the plunger or tool used for forcing the composite blank through the extrusion orifice. The tool typically is shaped to form a welding nib on the end of the electrode for use in attaching a terminal wire. The shaped tool has a limited life and, consequently, the machine extruding the composite electrode must be shut down frequently for maintenance to replace the tool.